1. Field
The following description relates to subsurface survey, and more particularly, to a technique for imaging a subsurface through signal processing using waveform inversion.
2. Description of the Related Art
Waveform inversion is a technique of estimating a velocity model of the subsurface using prestack seismic data.
The waveform inversion means a series of processes for obtaining a subsurface structure model close to the actual subsurface structure of an area of interest by creating an initial model for the area of interest, obtaining observed wavefield on the area of interest, and iteratively updating the initial model using the observed wavefield. The processes are carried out by calculating modeled wavefield from an arbitrary subsurface using a computer and iteratively updating parameters representing the physical characteristics of the subsurface such that the differences of the theoretical values and data obtained through actual field exploration are minimized.
Waveform inversion is one method of analyzing the subsurface structure of an area, which is one of the goals of geophysical exploration, and various mathematical methods are suggested. One of representative mathematical methods for waveform inversion is an ‘iterative least-squares method’. Generally, a unique solution does not exist in waveform inversion. Thus, a method of obtaining an optimal solution by adding a particular condition is used in waveform inversion. In this case, it is optional whether to give weight to convergence or to obtain a more accurate solution from a given measured data. Since an inverse model must be simplified and often requires extreme assumptions, it is important in waveform inversion to make the most of prior geological information related to geophysical characteristics.
One of the most important geological and physical characteristics for accurate subsurface imaging is the seismic propagation velocity of the subsurface medium. Recently, studies into a technique of artificially exploding a source wavelet into a target area, measuring a seismic wave against the target area and performing waveform inversion in a time domain or frequency domain on seismic data, i.e., data on a seismic wave reflected or refracted by the target area to obtain a seismic propagation velocity with respect to the subsurface medium are actively going on.